Technical Field
The present invention relates generally to liquid crystalline light modulating devices, and more specifically to new polymer free liquid crystalline display cells and materials which exhibit different optical states under different electrical field conditions and are characterized by a unique combination of properties, including optical multistability and haze-free light transmission at all viewing angles in both a field-ON or field-OFF mode.
Electrically switchable liquid crystal films intended for use in electrooptical devices have been prepared using various types and concentrations of liquid crystal and polymer. One such technique involves imbibing liquid crystal into micropores of a plastic or glass sheet. Another technique involves evaporation of water from an aqueous emulsion of nematic liquid crystal in a solution of water-soluble polymer such as polyvinyl alcohol or in a latex emulsion.
A different procedure offering significant advantages over mechanical entrapment techniques and the emulsification procedure involves phase separation of liquid crystal from a homogeneous solution with a suitable synthetic resin to form a liquid crystal phase interspersed with a polymer phase. These types of films, some of which are referred to as PDLC, have been shown to be useful in many applications ranging from large area displays and switchable coatings for windows to projection displays and high-definition television.
All of the above-noted materials and procedures have the disadvantage of requiring numerous and expensive reagents and starting materials. The various imbibing, emulsification or polymerization procedures associated with these systems significantly add to the cost and complexity of their manufacture. Moreover, when significant amounts of polymer are used, they begin to exhibit the characteristic drawback of "haze" at increasing oblique viewing angles until an essentially opaque appearance is detected at an oblique enough angle due to the perceived mismatch between the effective index of refraction of the liquid crystal and the refractive index of the polymer.
In the parent application it was found that good color reflective displays could be prepared using chiral nematic liquid crystal and polymer. These displays had the advantages of exhibiting multiple stable color reflecting states and, when the amount of polymer was low, haze free viewing. However, in spite of their many advantages, these displays still require the use of polymers and hence, have the drawbacks associated therewith.
Surprisingly, it has now been discovered that a polymer free multistable color reflecting cell can be prepared that exhibits stable color reflecting and light scattering states with multiple stable optical states therebetween characterized by varying degrees of intensity of reflection. Depending upon the voltage of the electric field addressing pulse, the material can be switched between these multiple optical states, all of which are stable in the absence of an applied field.